We welcome the efforts by the ICRU committee to develop a better framework for the operational quantities in radiation protection, aimed at addressing some of the current limitations and potential underestimation of the effective dose in certain fields (e.g. photon beams > 5-10 MeV).
Although the proposed new quantities, such as the ambient dose H*, personal dose Hp, and personal absorbed dose in local skin, represent a simplification in comparison to the current system, our main criticism is that “the impact of changes on routine measurement practice, including instrument design and calibration” have NOT been properly considered. It concerns us in particular that the new quantities are likely not physically realizable for photons in a personal dosimeter. (In the case of neutrons it can be argued that it was already not the case with the old recommendations anyway.)
The ICRU recommendations are not just a theoretical framework; they have direct influence in adopted technical requirements for practical radiation dosimeters, which should not only be technically feasible, but also cost effective and wearable.
In the case of the old recommendations, Hp(10) and Hp(0.07) are in principle physically realizable for photons if one assumes a tissue equivalent detector material and, in the case of Hp(10), a filter to shift the depth of measurement. In these approximate conditions, the detector response (which is proportional to the absorbed dose) would in principle follow closely the dose equivalent and, by definition, Hp(10) or Hp(0.07).
In the new definitions, that is not the case anymore and one needs to perform a crude spectrometry using a few detector elements and an associated algorithm. Although this is in principle feasible with a few existing detector systems, it is not clear how complex the dosimeter would have to be to satisfy the new requirements for different angles. This will most likely be impossible to achieve using a combination of detectors and flat filters, and will require possibly extremely complex dosimeter designs, if feasible at all.
For higher photon energies (> 1 MeV), it will also be a challenge to obtain “spectrometry” information for high photon energies, since one would need very dense or thick filters to obtain a different response in two different detector elements.
Therefore, we support the suggestion by other authors who commented on these recommendations (e.g. Markus Figel, Individual Monitoring Service, Helmholtz Zentrum Munich) that the ICRU Committee demonstrates that the new operational quantities are in principle realizable, or that changes to the current recommendations be postponed until sufficient scientific information exist to support the implementation of the new recommendations, and until we better understand their impact on routine measurements. At a minimum, the ICRU recommendations must provide better guidance on how these operational quantities are to be implemented in practice.
Head of the Dosimetry Group
Paul Scherrer Institute